Re: [PATCH bpf-next] libbpf: add xsk_umem__adjust_offset

["Björn Töpel" <bjorn.topel@gmail.com>]

On Thu, 12 Sep 2019 at 17:47, Kevin Laatz <kevin.laatz@intel.com> wrote:
>
> Currently, xsk_umem_adjust_offset exists as a kernel internal function.
> This patch adds xsk_umem__adjust_offset to libbpf so that it can be used
> from userspace. This will take the responsibility of properly storing the
> offset away from the application, making it less error prone.
>
> Since xsk_umem__adjust_offset is called on a per-packet basis, we need to
> inline the function to avoid any performance regressions.  In order to
> inline xsk_umem__adjust_offset, we need to add it to xsk.h. Unfortunately
> this means that we can't dereference the xsk_umem_config struct directly
> since it is defined only in xsk.c. We therefore add an extra API to return
> the flags field to the user from the structure, and have the inline
> function use this flags field directly.
>

Can you expand this to a series, with an additional patch where these
functions are used in XDP socket sample application, so it's more
clear for users?


Thanks,
Björn

> Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
> ---
>  tools/lib/bpf/libbpf.map |  1 +
>  tools/lib/bpf/xsk.c      |  5 +++++
>  tools/lib/bpf/xsk.h      | 14 ++++++++++++++
>  3 files changed, 20 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index d04c7cb623ed..760350c9b81c 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -189,4 +189,5 @@ LIBBPF_0.0.4 {
>  LIBBPF_0.0.5 {
>         global:
>                 bpf_btf_get_next_id;
> +               xsk_umem__get_flags;
>  } LIBBPF_0.0.4;
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index 842c4fd55859..a4250a721ea6 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -84,6 +84,11 @@ int xsk_socket__fd(const struct xsk_socket *xsk)
>         return xsk ? xsk->fd : -EINVAL;
>  }
>
> +__u32 xsk_umem__get_flags(struct xsk_umem *umem)
> +{
> +       return umem->config.flags;
> +}
> +
>  static bool xsk_page_aligned(void *buffer)
>  {
>         unsigned long addr = (unsigned long)buffer;
> diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
> index 584f6820a639..bf782facb274 100644
> --- a/tools/lib/bpf/xsk.h
> +++ b/tools/lib/bpf/xsk.h
> @@ -183,8 +183,22 @@ static inline __u64 xsk_umem__add_offset_to_addr(__u64 addr)
>         return xsk_umem__extract_addr(addr) + xsk_umem__extract_offset(addr);
>  }
>
> +/* Handle the offset appropriately depending on aligned or unaligned mode.
> + * For unaligned mode, we store the offset in the upper 16-bits of the address.
> + * For aligned mode, we simply add the offset to the address.
> + */
> +static inline __u64 xsk_umem__adjust_offset(__u32 umem_flags, __u64 addr,
> +                                           __u64 offset)
> +{
> +       if (umem_flags & XDP_UMEM_UNALIGNED_CHUNK_FLAG)
> +               return addr + (offset << XSK_UNALIGNED_BUF_OFFSET_SHIFT);
> +       else
> +               return addr + offset;
> +}
> +
>  LIBBPF_API int xsk_umem__fd(const struct xsk_umem *umem);
>  LIBBPF_API int xsk_socket__fd(const struct xsk_socket *xsk);
> +LIBBPF_API __u32 xsk_umem__get_flags(struct xsk_umem *umem);
>
>  #define XSK_RING_CONS__DEFAULT_NUM_DESCS      2048
>  #define XSK_RING_PROD__DEFAULT_NUM_DESCS      2048
> --
> 2.17.1
>